Need advice

So I have this best friend that I have been involve with ever since we met and we dated once when we were smaller but had to leave me because his ex was pregnant and I understood so I held no grudge towards him was there for him whenever and our relationship turned into becoming best friends and we had Been best friends for 6 or 7 years and now we have 3 months together as a couple and his baby mama isn’t in the picture not even with her own daughter ,his daughter is 3 and we are both 19 we going to be 20 soon and honestly I love him and all and he does to but it’s his first time being with someone else who isn’t the mom of the child but i honestly just need some advice, sometimes idk what to say or how to be around him and the baby like I enjoy being around kids and all and I buy the little girl stuff and treat her like if she’s my own we both want long term and all I just feel like I don’t know how to be around them both or talk to him about her he says it doesn’t bug him that I treat her as my own just that he just wants to make sure how I am and if ima stick around for a long run before I get to spend more time with him and his baby I only seen the little one 3 times his family isn’t aware we are dating but has asked about me and what I am to him and he says to them don’t worry about it like idk how I should feel honestly but I feel some type of way when he says those types of things like I know it’s not easy to trust but idk I need advice anything helps